Week 10 - visual dysfunction

• dyslexia entails issues reading and sequencing events. Problem with magnocellular part of the visual system (dorsal, where)
• dyslexics are less able to see coherent motion amongst random motion
• the deficit of coherent motion isnt large - they need about 3% extra coherent dots to spot the coherent nature
• true dyslexia is characterised by poor temporal processing caused by impaired development of magnocellular systems through the brain
• schizophrenia characterised by disorganised thinking and speech, poor memory, poor auditory, social and emotional function
• autism causes a lack of social awareness as well as heightened sensory awareness with excessive attention to detail and sensitivity to change
• autistic children show significantly higher motion coherence thresholds then normal children - means they need more coherence to spot the trend
• all 3 conditions effected by motion coherence but doesn't mean they have the same cause
• dorsal system including the superior temporal sulcus has a role in processing motion, event timing and social stimuli. This is disrupted in all of these conditions making this a possible explanation for poor motion coherence
• stroboscopic flashing in the range of 3-60 Hz peaking in the population at 15 Hz is one visual trigger of epilepsy
• striped patterns 0.2- 10 cycles per degree of visual angle, peaking in the population at 2.5 c/deg
• in EEG there is evidence of cortical excitation in patients with photosensitive epilepsy suggesting this may predict the fits
• migraines are severe prolonged headaches with a tendency for nausea and may be preceded with an aura which may be visual
• one can have an aura without a headache
• visual triggers 'stimuli' cause migraines
• very bad patterned stimuli for epileptic individuals is also highly uncomfortable for migraine sufferers in general
• visual stress triggers excitation in cortex- cortex fails to inhibit excessive excitation causing neurons to get fatigued (which inhibition is aiming to avoid)- neurons run out of oxygen causing the brains blood vessels to dilate and deplete oxygen causing excessive headaches
• migraine aura is a 'hole' in the fovea followed by shimmering, oriented coloured lines
• visual stress or 'meares-irelen syndrome' include finding text difficult to read, eyes strained, uncomfortable and difficulty concentrating
• treatment of visual stress includes a full eye test to rule out optical errors, then using coloured overlays for reading and coloured paper, wearing precision tint lens, colour overlays have to be very precise and different for each patient
• changing stripe colours may redistribute the activity away from over sensitive (local) areas of cortex
• fMRI evidence shows cortical activity is reduced in areas V3 (decodes motion) / V4 (decodes colour) in migraineurs as well as visual stressers
• there is some evidence against the visual stress argument such as symptoms being very broad, large scope for placebo, treatments being very general for many conditions etc
• there is also some evidence for visual stress such as evidence supporting the theory, rates of reading showing immediate improvements, EEG and fMRI evidence etc
• classical bottom up view of vision includes processing starting with the stimulus on the retina and works via increasingly complex stages in ever higher brain areas towards some behavioural goal
• in object recognition you should segment the object from the background then identify contours bulding these into outlines and then recognise the object
• separating the object from background is figure ground segmentation. Contrast sensitive neurons in each sub system are responsible for this.
•  - Retinal ganglion / LGN cells find object edges
• Orientation-contrast -> texture segmentation,
• Motion-contrast -> figure-ground for motion,
• Disparity neurons -> direct object segmentation.
• sometimes bottom up processes are not enough so top down may also be needed to assign the figure
• illusory contours is when you perceive a contour when no line exists. This develops at around 8 m for static displays, 2-3 m for moving
• V2 is the area which is responsible for illusory contours
• hyper complex cells in V1 on either side of the illusory contour are inhibited by the extended lines, whereas those at the end of the real lines are less inhibited. So V1 detects the line endings passing this signal to cells in V2. Activity in the centre gives the impression that there is activity on the edge when there isnt.

  
• the ability to link short sections of a contour to make a longer contour is contour integration
• Contour integration is the opposite of length sensitivity in hyper complex cells. Thought to come about due to long range excitatory connections between cells that prefer similar orientations but at different positions
• contour integration takes place within V1 and develops at 2 1/2
• form cells in V4 respond strongest to stimuli with a flick on one side
• neurons in V4 respond to more complex features than earlier visual areas, but at a level less complex than whole objects. There is a big role for attention in V4 (top down)